250 THE ORIGIN OF THE NERVOUS SYSTEM 



posterior end over more anterior levels of the secondary 

 gradient appears very clearly in the respiratory move- 

 ment of the posterior body region when the animals are 

 undisturbed. This movement of the body to and fro in 

 the water is initiated at the posterior end, and that it is 

 independent of the head is evident from the fact that it 

 may occur in headless animals. But although this poste- 

 rior region is ordinarily largely independent of the head, 

 and the impulses in it apparently run predominantly in 

 the anterior direction, a sufficient stimulation of the head, 

 or in fact of any region of the body, brings this region 

 more or less under the control of the head, the respira- 

 tory movement is inhibited, and the animals may move 

 in a more or less co-ordinate manner. In these forms, 

 then, the functional directions of the chief arcs are 

 apparently opposite in anterior and posterior regions 

 under conditions of slight stimulation but with suffi- 

 ciently intense excitation impulses may pass in either 

 direction in all regions. In other words, the functional 

 direction of the arcs which predominate under conditions 

 of slight stimulation is down the gradients in both 

 directions, but with strong stimulation impulses are 

 conducted up as well as down. Paths upward are 

 present, but a high degree of excitation is apparently 

 necessary to make them available. To what extent or 

 whether at all the upward and downward paths are iden- 

 tical is not known, though it appears not improbable that 

 they may be identical over some portions of their length. 



FUNCTIONAL DIRECTION IN THE HIGHER ANIMALS 



While any extended survey of the conditions in the 

 various groups is impossible at this time, it may be 

 noted that with the progress of evolution the relation 



